The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, is the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or members that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels", "superabsorbents", "xerogels" or "hydrocolloids" has been particularly important. See for example, U.S. Pat. No. 3,699,103 (Harper et al.), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such materials (hereinafter referred to as "absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See for example, U.S. Pat. No. 4,673,402 (Weisman et al.), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al.), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or poly-functional monomers such as N,N'-methylene-bisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to lightly cross-link the polymer chains thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked absorbent polymers contain a multiplicity of carboxyl groups attached to the polymer backbone. These carboxyl groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network. Absorbent polymers can also be made by polymerizing unsaturated amines or derivatives thereof in the presence of relatively small amounts of di- or poly-functional monomers, in an analogous fashion.
The degree of cross-linking of these absorbent polymers is an important factor in establishing their absorbent capacity and gel strength. Absorbent polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength relates to the tendency of the swollen polymer particles to deform under an applied stress, and needs to be such that the particles do not deform and fill the capillary void spaces in the absorbent member or article to an unacceptable degree, thereby inhibiting the rate of fluid uptake or the fluid distribution by the member or article. In general, increasing gel strength will result in an increase in the permeability of a zone or layer comprising swollen absorbent polymer. However, this typically also reduces the absorbent capacity of the gel undesirably. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al.), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. No. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al.), issued May 30, 1989.
Many absorbent polymers can exhibit gel blocking under certain conditions. "Gel blocking" occurs when particles of the absorbent polymer deform so as to fill the capillary void spaces in the absorbent member or article to an unacceptable degree, thereby inhibiting the rate of fluid uptake or the distribution of fluid by the member/article. Once gel-blocking occurs, further fluid uptake or distribution takes place via a very slow diffusion process. In practical terms, this means that gel-blocking can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent member or article. Leakage from the absorbent article can take place well before the particles of absorbent polymer in the absorbent article are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent article. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989
This gel blocking phenomenon has typically necessitated the use of a fibrous matrix in which are dispersed the particles of absorbent polymer. This fibrous matrix keeps the particles of absorbent polymer separated from one another and provides a capillary structure that allows fluid to reach the absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can significantly increase the bulkiness of the absorbent article or lower the overall fluid storage capacity of thinner absorbent structures. Using low concentrations of absorbent polymers limits somewhat the real advantage of these materials, i.e. their ability to absorb and retain large quantities of body fluids per given volume.
Another factor which affects the transport of fluid in an absorbent member is the integrity of the region or regions that comprise these particles of absorbent polymer. Such region or regions having the high concentration of absorbent polymer should have sufficient integrity in a partially wet and/or wetted state such that the physical continuity (and thus the capability of acquiring and transporting fluid into and through contiguous interstitial voids/capillaries) of the absorbent member is not substantially disrupted or altered when subjected to normal use conditions. During normal use, absorbent cores in absorbent articles are typically subjected to tensional and torsional forces of varying intensity and direction. These tensional and torsional forces include bunching in the crotch area, stretching and twisting forces as the person wearing the absorbent article walks, squats, bends, and the like. If wet integrity is inadequate, these tensional and torsional forces can potentially cause a substantial alteration and/or disruption in the physical continuity of the absorbent member such that its capability of acquiring and transporting fluids into and through the contiguous voids and capillaries is degraded. The layer comprising absorbent polymer can be partially separated, be fully separated, have gaps introduced, have areas that are significantly thinned, and/or be broken up into a plurality of significantly smaller segments.
The integrity of a swollen layer of the absorbent polymer can be increased by various techniques . For example, the particles of absorbent polymer may be coated with a second polymer that has a strong affinity for the polymer comprising the absorbent particles. When aqueous fluid is introduced to such a system, this second polymer can bridge between the absorbent polymer particles to form a three-dimensional network of particles bound together by polymer chains. This technique is described in greater detail, for example, in U.S. Pat. No. 5,382,610, filed Jan. 17, 1995, in which conventional absorbent polymer particles comprising poly(acrylic acid) are coated with polyethylenimine to yield swollen polymer matrices with good integrity. However, although the integrity of the swollen polymer can be increased by this technique, the absorbent capacity of the system under normal use conditions is generally depressed. Furthermore, the integrity of the swollen polymer matrix tends to deteriorate with time, as the uncrosslinked polymer which binds the particles together is able to diffuse away from the surfaces of the polymer particles.
Accordingly, it would be desirable to provide an absorbent polymer composition capable of absorbing a large quantity of a synthetic urine solution. It would also be desirable that the region or regions in the absorbent member comprising these absorbent polymers have sufficient integrity in a partially wet and/or highly swollen state such that the physical continuity (and thus the capability of acquiring and transporting fluid through contiguous interstitial voids/capillaries) is not substantially disrupted or altered, even when subjected to normal use conditions.